1. Field of the Invention
The present invention relates to a machine tool provided with a tool spindle head which is pivotable and to which a tool is to be mounted, such as a composite lathe.
2. Description of Related Art
Recently, there has been developed a composite lathe which is capable of performing machining for forming e.g. an oblique hole in addition to milling and drilling operations. Such a composite lathe includes a tool spindle head to which various kinds of tools stored in a tool magazine can be removably mounted by an automatic tool changer (ATC).
Examples of such composite lathe include a double-spindle machining lathe 1 having such a structure as shown in FIG. 9. Specifically, the double-spindle composite lathe 1 includes a first headstock 12 fixedly provided on one side of a bed 11 and a second headstock 13 provided on the other side of the bed 11 movably in the Z-axis direction in facing relationship to the first headstock 12. The composite lathe 1 further includes a saddle 16 provided between the first headstock 12 and the second headstock 13 on the bed 11 movably in the Z-axis direction, a tool rest 15 mounted on the saddle 16 movably in the X-axis direction and supporting a turret 14 for allowing the indexing movement, and a column 17 standing on the rear side of the bed 11 movably in the Z-axis direction on the bed 11. The column 17 has an inclined surface to which a tool spindle head 19 is mounted pivotally in the B-axis direction via a support base 18 which is movable in the X-axis direction.
In the composite lathe 1, by pivoting the tool spindle head 19 in the B-axis direction, a tool spindle can be so set as to makes an arbitrary angle with respect to the main spindle. Therefore, by mounting a hob as a gear cutting tool to the tool spindle head 19, a workpiece set to the first headstock 12 and the second headstock 13 can be subjected to gear cutting as well as the machining to form an oblique hole. Therefore, the composite lathe is advantageous in that it is unnecessary to subject a workpiece to pre-machining such as cutting and then perform gear cutting by setting the workpiece to a hobbing machine designed for gear cutting only.
During the gear cutting by using the composite lathe 1, the hob is held by the tool spindle head 19 only at one side thereof. However, such gear cutting performed while holding the hob only at one side thereof is likely to warp the hob, because the cutting resistance of the hob relative to the workpiece is high. Further, due to vibration and change of cutting resistance during the machining, it is difficult to achieve sufficient workpiece machining accuracy.
To solve such problems, the applicant of the present invention has proposed a composite lathe, which is disclosed in Japan Patent Laid-Open Publication No. 2003-311502 and illustrated in FIGS. 10 and 11 of the present specification. As shown in the figure, the lathe includes a tail-stock type tool supporting unit 50 mounted to a turret 14. The tool supporting unit 50 includes a main portion 52 supported by a rest 51 pivotally in the B-axis direction and provided with a tailstock spindle 53 which is capable of advancing and retreating due to hydraulic pressure. One side of a hob H, which is an example of tool which can be supported at opposite sides thereof, is mounted to a tool spindle head 19, and the other side of the hob H is pivotally supported by the tailstock spindle 53 of the main portion 52. It is to be noted that the reference sign W in FIGS. 10 and 11 indicates a workpiece.
However, in the above tool supporting unit 50, the main portion 52 including the tailstock spindle 53 for supporting the hob H merely pivots in the B-axis direction. Therefore, to perform gear cutting with respect to workpieces of different diameters, the tool supporting unit 50 needs to be changed in accordance with the diameter of each workpiece. Further, even in performing gear cutting with respect to workpieces of the same diameter, the mounting position of the tool supporting unit 50 needs to be adjusted in accordance with the amount of wear when the hob H is worn, which causes a deterioration in working efficiency.